1. Field of the Invention
This invention relates to the field of capacitors with a long failure time, and more specifically this invention relates to a ceramic capacitor which fails gracefully so as to exhibit a gradual a loss of capacitance.
2. Background of the Invention
A capacitor is used to store electrical charge, or, equivalently, electrical energy. A capacitor can also function as a filter by passing alternating current (AC) while blocking direct current (DC). Capacitors also can serve as a source of instantaneous released energy, or to prevent current and voltage transients across batteries. Typically, a capacitor comprises two electrode plates facing each other, with an insulating dielectric between the plates.
Prior art capacitors are prone to catastrophic failure, especially when the dielectric is a ceramic. At high voltages, dielectric breakdown can occur due to either extrinsic effects (e.g. material defects or porosity), or intrinsic effects (e.g. thermal runaway). When breakdown occurs, there is an in-rush of current across the defect region causing a short circuit across the capacitor. After breakdown the entire capacitor can no longer hold charge and cannot function as a charge storage device. For many applications, this short circuit is harmful to other components in the circuit and can result in catastrophic damage. Efforts have been made to minimize catastrophic damage to circuit components due to capacitor failure. Capacitors which fail “gracefully” instead of catastrophically have been attempted. Capacitors which exhibit graceful failure ideally undergo a miniscule loss of capacitance when a failure occurs. The objective is to allow several isolated failure events to occur before an appreciable loss of capacitance is experienced. This would confer better circuit stability, and extend useful life for the circuit.
Graceful failure has been attempted in metallized polymer capacitors. These designs comprise a polymer film on both sides of which thin layers of metal are deposited. The polymer film provides mechanical support for the two electrodes. (See U.S. Pat. No. 4,433,359 to Hanabe et al.)
Typical ceramic capacitors have dielectrics on the order of 1 micron thick with metal electrodes of the same thickness. Ceramic dielectrics are preferable for use in many capacitors applications because they have much higher dielectric constants and therefore higher energy densities than polymer capacitors. Air exhibits a dielectric constant (also known as “permeability”) very close to 1. Polymers have a relative permeability (relative to air) of between 1 and 6. Ceramic dielectrics, particularly ferroelectric materials, have a relative permeability of between 30 and 100,000.
The high dielectric constant of ceramic capacitors makes them particularly suitable in high frequency applications such as decoupling capacitors in microelectronics and filters in mobile communication. Capacitors with ceramic dielectrics can be operated at higher temperatures than polymer capacitors. Additionally, they can accommodate higher ripple currents. Ceramic capacitors are also better suited for high frequency applications.
Graceful failure has not been extensively shown in ceramic capacitors. This is because ceramic dielectrics of a thickness similar to that of a typical polymer capacitor are too brittle to provide the support structure for a capacitor. Also, the higher dielectric capacities of ceramics result in higher charges being stored, therefore increasing the potential for short circuiting or other failure.
A few designs have demonstrated graceful failure for ceramic capacitors but such designs rely on an internal thin metal self-fusing strip. This design mimics fusing strips in certain polymer capacitor designs. These strips are not easily amenable to large-scale manufacturing as they rely on the use of a thin segment often not fabricated in a similar manner as the main bodies of the electrodes. (See for instance U.S. Pat. No. 4,894,746 to Mori et al., U.S. Pat. No. 4,720,767 to Chan et al., and U.S. Pat. No. 4,680,670 to Chan.)
FIG. 1 illustrates in principle the state of the art electrode configuration for a capacitor having fusing strips. It duplicates in part FIGS. 5A and 5B in U.S. Pat. No. 4,720,767. An electrode 11 (placed below electrode 14) comprises two electrode segments 13a and 13b overlying dielectric material “D”. The electrodes 13a, 13b are connected by fusing strips 19a, 19b to voltage terminals 33a and 33b which are at the same potential. When a voltage breakdown occurs between electrodes 14 and 11 (say above 13a), there is a short-circuit across the capacitor and therefore a surge of current in the corresponding electrode segment 13a such that the connecting fusing strip 19a is melted. This disconnects the whole electrode segment 13a where the breakdown occurred from the voltage terminal 33a. Consequently, since one of two electrode segments is disconnected, an appreciable and significant loss of capacitance occurs.
A need exists in the art for an improved ceramic capacitor which does not fail short and result in a short circuit. The capacitor should obviate the need for fusing strips and other intricate electrode designs and should instead incorporate uniform electrode configurations to minimize the cost of fabrication. The electrodes should be such that only the localized, defective area of an electrode, and not the entire electrode, is removed from the capacitor structure, so as to yield only a minuscule (i.e., nearly undetectable) loss of capacitance when voltage breakdown occurs.